Okay my friends. Thank you very much for your messages! I've gone the difficult way and removed the bench. I was able to open the trunk from inside. I've read it is very common to "loose" the pin in the lock mechanism - same situation for me. The pin is lost, so this caused my lock to not open anymore. I think it's stupid to just buy a pin instead of a new lock.
Anyway, again, thank you guys for helping me! I might upload a step-by-step thread with pics on disassembly the rear bench if needed.
